Android sdk img verify

[moondial-pal]

Adds a list_installed_system_images() utility method to AndroidSDK that invokes sdkmanager --list_installed and returns a set of installed system image package identifiers. Updates verify_system_image() to use this method instead of only checking if the system image directory exists.

Previously Briefcase verified a system image by checking if the expected directory existed. This check is not strong enough as the directory may exist wile the system image is incomplete or corrupted. This would cause a confusing error when either avdmanager or the emulator would try to use an incomplete system image.

Fixes #1895

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[moondial-pal]

The list_installed_system_images() method partially overlaps with #737. It seems that ticket will require a similar method using sdkmanager --list to get all available images, along with additional parsing to present meaningful choices to the user.